A TiAl intermetallic compound-base alloy material is included among materials under development for use as structural materials having environmental resistance. Since this material has excellent strength properties at high temperatures, the development as a structural material for use at high temperatures in the future is expected in the art. Regarding this material, attention has been drawn to the strength at high temperatures which is comparable to the property values of the conventional Ni-base and Co-base superalloys. Further, it should be noted that the specific gravity of the TiAl intermetallic compound-base alloy material is 3.8 while the specific gravity of the superalloys is close to 10. When this fact is taken into consideration, the TiAl intermetallic compound-base alloy material is superior to the superalloys in specific strength at high temperatures. Therefore, it is a promissing material for advanced airplanes which should be lightweight.
However, the upper limit of the service temperature of alloy materials having strength at high temperatures, such as these superalloys, including TiAl intermetallic compound-base alloy materials is 900.degree. C., and metallic materials having satisfactory strength at 900 to 1100.degree. C. have not been developed in the art. Rather, nonmetallic materials, such as ceramics and C/C (carbon/carbon fibers), are used in temperatures of 1000.degree. C. or above. These nonmetallic materials has high strength at high temperatures. However, fracture, in most cases, is created within elastic stress, so that the ductility is zero. For this reason, the development of alloy materials having ductility has been desired from the viewpoint of safety.
In the prior art, there is a near net shape casting technique where an intermetallic compound sheet is produced. The technique for producing a near net shape sheet has been rapidly advanced in recent years. In particular, in the metallic materials, the advance in the application to the production of stainless steel sheets is significant. Various casting process have been proposed for the production of the sheet. Among them, a twin roll process is suitable for the production of a continuous sheet having an even thickness.
A nickel-aluminum intermetallic compound (Ni.sub.3 Al) having ductility improved by the addition of a very small amount of boron is known as an example of the application of the above technique to intermetallic compound materials. This has been reported in an international conference concerning "Casting of Near Net Shape Products" held in November 1988 (Proceeding of an International Symposium of Near Net Shape Products, pp. 315-333, issued by The Metallurgical Society). Further, a process for producing a TiAl intermetallic compound sheet is described in Japanese Patent Application No. 501367/1991.
Further, for TiAl intermetallic compound materials with boron added thereto, U.S. Pat. No. 4,842,820 discloses a production process utilizing plasma melt process and isothermal forging, and U.S. Pat. No. 4,751,048 discloses a production process utilizing mechanical alloying.
For TiAl intermetallic compound materials with chromium added thereto, U.S. Pat. Nos. 4,842,819 and 4,879,092 disclose a production process utilizing plasma melt process and isothermal forging.